This invention relates to blown film production, and more particularly to tubular film process and apparatus, wherein from molten thermoplastic material, or plastic, is extruded or die formed a tubular film which is drawn out over and expanded by a volume of compressed air, which is contained within an appropriate length of the tubing by the flatting of the tube for wind-up on supply rolls. In this blown or tubular process the size of the tube, or width of the ultimate wound roll, is controlled by admitting or extracting air to raise or lower the pressure by which the air bubble expands the plastic, before it is chilled or cooled from the molten to the solid state, and the wall or gauge of the tube is drawn down or attenuated from the die opening to the desired thickness by control of the surface speed of the tube flatting or nip rolls relative to the rate at which the thermoplastic material is extruded through the die.
This invention is concerned more particularly with the production of multi-wall or laminated film, and still more particularly with film laminated from unlike or dissimilar material, known as composite film.
The laminating of films by the blown film process here concerned is distinguished from the mechanical laminations, such as variously known as adhesive lamination, wet lamination, heat lamination, extrusion coating, or hot melt.
Whereas in the mechanical laminations two or more operations (or a number of steps in one operation) are required, in the blown film process the dissimilar materials are combined simultaneously, by extrusion through a common die.
The structuring of the multi-layered composite by blown film extrusion as here concerned brings the two or more layers of thermoplastic materials or polymers into contact while still in an amorphous or semi-molten state, and yields an intimately layered composite which to the eye is indistinguishable from monofilm. The composite film layers are thus bonded together without adhesive, and at less cost than a mechanical laminate of a plurality of film layers.
By the use of the multi-layered extrusion there is also obtained far greater control of the film, and adaptability of it to different end uses, such as in packaging. The greater control and adaptability is obtained by determination of the thickness and location (outer, intermediate, inner) of, and as well by the selection of the material for, the two or more film layers, or plies.
Thus, and for example, a three layered construction may typically comprise inner and outer laminations of polyethylene, and an intermediate or liner film or ply of a more costly, or exotic nature (nylon, saran, polypropylene, ionomers, EVA, H.D.P.E., polyester, rubber modifieds, etc.) This combining of different materials will be understood to supply one or another property wanted for the composite, such as gas barrier, puncture resistance, deep-draw flex, etc.
An important advantage of the blown film composite just described is the elimination of the need for shifting resins from one extruder to the other, in going from the laminate on the extreme inside to that on the extreme outside.
The advantages of the blown film method, over the mechanical methods of producing composite multi-layer structures, more generally comprise, among others, simplicity and economy equipment, flexibility of gauge and width, the capability of uniting resins that can be effectively combined only in the molten state, and the complete elimination of any oxidation taking place between film surfaces.
The blown film method here concerned will be understood to afford a tubular structure, or any combination thereof such as sheeting. U-film, J-film, or center fold, which cannot be achieved by mechanical process.
The blown laminating film process where concerned affords more particularly the extrusion of free films which cannot be achieved effectively by free blown method. Thus in the blown laminating film process a support tubing type material can be added, satisfactorily, as a lining type material, such as for example cellulose acetate, and others.
Among still other advantages of the blown film process there are herein to be mentioned the far greater manufacturing flexibility as to film gauges and widths, and correspondingly greater adaptability to changes in customer requirements in those regards.
Still further advantages known to be derived from the blown film process are the enhancement of the film properties, as by the molecular orientation in both the machine and the transverse directions, and by the control of the melt temperature for, and the elimination of pin holes oxidation, and other defects in, the film forming.
With respect to the orienting, for improved optical and other film properties, it is to be noted that the biaxial orienting herein obtained is not achievable with mechanical laminating, except by an impractical second process. Also that in the blown extrusion process all the film layers are biaxially oriented, whereas in the mechanical process only the substrate may be so oriented, and that, as just mentioned, only by an uneconomic second stage, and with the coating layer left unoriented.
The blown film process hereof will also be understood to afford the imparting of the biaxial orientation to, and thereby to bring out desired properties in each layer of the composite film, thus yielding a combined or laminated film product far superior to that of the non-orienting or merely linearly orienting mechanical processes.